1. Field of the Invention
The present invention relates to a control system for an internal combustion engine having a cylinder halting mechanism for halting the operation of some of a plurality of cylinders by stopping the operation of intake valves of the cylinders, and particularly to a control system which performs a control for changing the number of operating cylinders in an idling condition of the engine.
2. Description of the Related Art
Japanese Patent Publication No. H2-22221 (JP-'221) discloses a control system wherein the intake air flow rate of the engine is changed when changing the number of operating cylinders in the idling condition of the engine. According to this control system, a signal for changing an opening of an intake air flow rate control valve is output when a predetermined time period has elapsed from the time a switching instruction signal for instructing a change in the number of operating cylinders is output. Therefore, the change timing of the intake air flow rate is delayed by a time period corresponding to a delay time period until the operating cylinder number is actually changed in response to the switching instruction signal of the operating cylinder number, so that the change timing of the operating cylinder number is made to coincide with the change timing of the intake air flow rate.
When performing a step change in the opening of the intake air flow rate control valve accompanied by the change in the operating cylinder number, as shown in JP-'221, there is a problem as described below.
In the operating condition where the operation of some of the cylinders is stopped, the operation of the intake valves of halting cylinders is stopped. Accordingly, the intake pressure at a portion of the intake pipe downstream of the throttle valve (intake air flow rate control valve) becomes higher than the pressure obtained when performing the all-cylinder operation. Upon switching from the partial-cylinder operation to the all-cylinder operation, the intake pressure at the portion downstream of the throttle valve does not immediately change even if the throttle valve opening is changed stepwise. Consequently, the intake air flow rate of the engine temporarily becomes excessive, so that the engine rotational speed in the idling condition greatly exceeds a target rotational speed and it takes a comparatively long time period until the engine rotational speed returns to the target rotational speed.